Although multi-area is definitely a scaling consideration, I am pretty certain that the backbone requirement (that all areas touch the backbone and exchange interarea routes only through the backbone) actually *is* for loop prevention. Since inter-area routing is distance-vector in nature, you need some way to avoid routing loops with the incomplete info inherent in a DV approach.
By requiring that any inter-area route passes through area 0, and that every other area touches area 0, you are ensuring the path goes *exactly* through the path area X, area 0, area Y. Since the respective routers in each of those areas has information about one end of the other of that path, you can't really have a route loop around from area Y back through the backbone and to area X or any other area again. If area Y was allowed to exchange routes directly with a non-backbone area Z, the route from X could get re-injected to the backbone again via Z and result in a loop. The strict requirement of all inter-area paths going through the backbone prevents this. (Transit capability is a very special case of breaking this requirement, but I'm ignoring that) Within an area there's essentially no risk of looping because of the nature of LS protocols and their complete knowledge of the area topology. At least that's my understanding. Bob On Tue, Jul 31, 2012 at 10:13 AM, <[email protected]> wrote: > OSPF forces a 2-tier hierarchy design with the backbone area requirment. > It was designed this way to allow the protocol to scale better than a flat > topology such as EIGRP. Your areas limit the LSA flooding scope and the > backbone area uses Type 3 summary to flood inter-area routes in a single > LSA. The area requirement is for scaling and is not a loop prevention > mechanism in itself. > Sent wirelessly from my BlackBerry device on the Bell network. > > -----Original Message----- > From: Chidhu R <[email protected]> > Sender: [email protected] > Date: Tue, 31 Jul 2012 12:47:33 > To: <[email protected]> > Subject: [OSL | CCIE_RS] Fwd: OSPF Backbone area rule > > I am aware of the basic facts and that inter area connection is a distance > vector logic. It would be greatly helpful if you can provide me an example > of how a loop will form when this condition is violated. > > Thanks > Chid > > ---------- Forwarded message ---------- > From: Chidhu R <[email protected]> > Date: Tue, Jul 31, 2012 at 12:27 PM > Subject: OSPF Backbone area rule > To: [email protected] > > > Hi, > > > > Can someone explain me, in OSPF, > > > > - Why every area must be connected to area 0 and why those routes in > areas which are not connected to area 0 are not being redistributed > across > the OSPF domain. ( which led virtual link concept i suppose ) > - Why any router which is an ABR should be connected to area 0 and why a > router which is not connected to area 0 but connected to multiple areas > do > not summarize the type 3 LSAs. > > > > > > Also please explain one more doubt in the picture attached with this > thread. When checked in R5, i am able to see all the routes including the > route > > > > between R1 and R2 ( *this is not connected to area 0, yet i am able to see > the routes and there is no Virtual link configured )* > > > > > > Thanks > > > > Chid > > _______________________________________________ > For more information regarding industry leading CCIE Lab training, please > visit www.ipexpert.com > > Are you a CCNP or CCIE and looking for a job? Check out > www.PlatinumPlacement.com > > http://onlinestudylist.com/mailman/listinfo/ccie_rs > _______________________________________________ For more information regarding industry leading CCIE Lab training, please visit www.ipexpert.com Are you a CCNP or CCIE and looking for a job? Check out www.PlatinumPlacement.com http://onlinestudylist.com/mailman/listinfo/ccie_rs
